In this study it was aimed to investigate the risk factors for gastrointestinal colonization by extended-spectrum beta-lactamase (ESBL)-producing Klebsiella pneumoniae and Escherichia coli in intensive care unit (ICU) of anaesthesiology and reanimation, Ege University Faculty of Medicine, Izmir, Turkey. This study was performed prospectively on adult patients hospitalized in ICU of anaesthesiology and reanimation and rectal swab cultures were performed in all patients in the first 48 hours of hospitalization and every one week until discharge or death. Samples were transported to the laboratory in Stuart transport medium and were cultured on two EMB agar plates (one including 4 mg/L ceftazidime) and incubated for 48 hours. E.coli and K.pneumoniae isolates were identified by conventional methods. Antibiotic susceptibility tests were performed by disc diffusion method on Mueller Hinton agar and interpreted according to CLSI guidelines. ESBL was confirmed by double disc synergy test. A total of 140 patients (49 female 91 male; age range: 18-83 years, mean age: 56.3 years) were evaluated, and 41(29.3%) of the patients were found to be colonized with ESBL positive E.coli (n=39) or K.pneumoniae (n=2). The mean time for colonization was 11.15 +/- 10.91 (range between 2-39) days. Age and gender of the patients and antibiotic consumption before or during the stay in ICU of anaesthesiology and reanimation were not found to be associated with colonization (p>0.05). However length of ICU of anaesthesiology and reanimation stay in colonized patients was longer than non-colonized patients (27.59 +/- 22.52 vs. 17.78 +/- 11.74 days; p<0.05). Infectious episodes developed in 22% (9/41) of the colonized cases and three of the nine strains were isolated from the blood cultures, five from the urine cultures and one from both blood and bronchoalveolar lavage cultures. ESBL-positive E.coli or K.pneumoniae colonization was found as an independent risk factor for the development of infection (9/41 vs. 4/99 cases; p=0.002). Forward logistic regression analysis revealed that diabetes mellitus, immunosuppresive drug use and length of intubation were associated with ESBL-positive E.coli or K.pneumoniae colonization (p<0.05). The results of this study indicated that the risk of development of infection was significantly high in intensive care patients colonized by ESBL positive E.coli and K.pneumoniae and the identification of the related risk factors was critically important for the establishment of effective control measures.